The use of liposomal delivery vehicles to facilitate the site-specific delivery of therapeutic agents represents a rapidly emerging field of drug delivery; however, the efficient delivery of therapeutic agents to targeted cells and tissues, as well as the subsequent transfection of such targeted cells and tissues remains a technical challenge. Despite the availability of multiple lipids and liposomal-based delivery systems to facilitate the delivery of therapeutic agents to target cells and tissues, many challenges still exist in both in vivo and in vitro applications. For example, a significant drawback of liposomal-based delivery systems relates to the construction of liposomes that have sufficient stability and the ability of such liposomes to efficiently release their encapsulated contents to targeted cells and tissues.
With respect to the development of liposomal delivery vehicles for use in delivering nucleic acids, the incorporation of cationic lipids as a component of a liposomal vehicle represents an important advancement. Properties of cationic liposomes, which include for example, their stability, size and surface charge, make them ideal carriers for encapsulating and delivering negatively charged nucleic acids to target cells and tissues. The cationic components (e.g., cationic lipids and/or cationic polymers) that comprise such cationic liposomal vehicles facilitate the interaction between the lipid bilayer of the liposome and the negatively charged nucleic acids, and thereby enhance the encapsulation efficiency of such cationic liposomal vehicles. In part due to their positive surface charge, liposomes prepared comprising cationic lipids (e.g., 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP)) have demonstrated an ability to be efficiently loaded with negatively charged nucleic acids and, the use of cationic liposomal vehicles may facilitate the encapsulation of nucleic acids at concentrations that well exceed those which would be achieved using neutral liposomal vehicles. For example, in certain instances, cationic lipid nanoparticle systems may be characterized as having encapsulation efficiencies approaching 100% when loaded with negatively charged nucleic acids. (See, e.g., Li, et al. Pharm Res., 2007, 24: 438-449.)
Many cationic lipids employed to construct such liposomal vehicles however, are generally toxic and accordingly, may be of limited utility, particularly in the quantities necessary to deliver therapeutically effective quantities of their encapsulated contents (e.g., nucleic acids). Further limiting the utility of charged liposomal systems, following their administration to a subject, such charged systems may be rapidly cleared from the systemic circulation and thereby limit their distribution and accumulation in targeted cells and tissues. To overcome the technical challenges associated with the use of cationic liposomal vehicles, the use of multi-component liposomal delivery systems has been employed. In particular, the preparation of liposomal vehicles using ionizable lipids has been employed as a means of modulating the charge of a liposome in response to the changing pH of an environment (e.g., physiological pH) to which the liposome is exposed. Such ionizable lipids therefore accommodate changes in their surface charge, which may be manipulated, for example, to enhance the encapsulation efficiency of the liposome.
Despite the foregoing limitations, and as a result of their ability to facilitate the delivery of encapsulated materials to target cells, liposomal-based vehicles are an attractive carrier for therapeutic agents and remain subject to continued development efforts. While liposomal-based vehicles that comprise a cationic lipid component have shown promising results with regards to encapsulation and stability, there remains a great need for improved liposomal-based delivery systems.
In contrast to charged liposomal-based vehicles, neutral liposomal vehicles are generally characterized as having relatively improved pharmacokinetic properties. However, in part due to the low encapsulation efficiency observed with neutral liposomes, there have been limited studies performed investigating the use of neutral liposomes to deliver therapeutic agents to target cells. There remains a need for novel lipids that incorporate a multi-functional approach for delivering encapsulated nucleic acids and polynucleotides. Particularly needed are lipid nanoparticles that retain some of the beneficial characteristics of both neutral and charged liposomal delivery systems.